Move main loop handling code to library_eventloop.js. NFC (#22615)

This means that programs that use the main event loops don't need to
depend on the `Browser` object.

We already supported and test the main loops handling code on node, but
programs that only use the event loop without any other `Browser`
functions no longer need to pull in the whole `Browser` object.

src/library_async.js

@@ -343,8 +343,8 @@ addToLibrary({
 #endif
           Asyncify.state = Asyncify.State.Rewinding;
           runAndAbortIfError(() => _asyncify_start_rewind(Asyncify.currData));
-          if (typeof Browser != 'undefined' && Browser.mainLoop.func) {
-            Browser.mainLoop.resume();
+          if (typeof MainLoop != 'undefined' && MainLoop.func) {
+            MainLoop.resume();
           }
           var asyncWasmReturnValue, isError = false;
           try {
@@ -391,8 +391,8 @@ addToLibrary({
 #if ASYNCIFY_DEBUG
           dbg(`ASYNCIFY: start unwind ${Asyncify.currData}`);
 #endif
-          if (typeof Browser != 'undefined' && Browser.mainLoop.func) {
-            Browser.mainLoop.pause();
+          if (typeof MainLoop != 'undefined' && MainLoop.func) {
+            MainLoop.pause();
           }
           runAndAbortIfError(() => _asyncify_start_unwind(Asyncify.currData));
         }